In order to protect the lives of passengers during a traffic accident, modern vehicles are generally provided with a protection system comprising several airbags and seat belt pretensioners, which are used to absorb the energy of a passenger released during the collision due to the accident. It is clear that such protection systems are most effective when they are well adapted to the specific requirements of actual seat occupancy. That is why microprocessor-controlled protection systems have been designed which provide several operational modes, allowing for example an adaptation of the instant at which airbags are deployed, the volume to which the airbags are inflated, the instant at which safety belts are released after the collision, etc, as a function of the stature of a passenger on the seat. In order to enable the control microprocessor to select the optimum operational mode for a given seat occupancy status, it is of course necessary to detect one or several parameters characterizing the occupancy status of the seat and to classify the occupancy into one of several classes, each of which is associated to a specific operational mode of the restraint system.
One approach for gathering relevant parameters of seat occupancy is based on the detection of the capacitive coupling of a body to one or several electrodes arranged in the seat. Such a measurement system is for instance described in LU-A-88 828. This measurement system comprises at least one transmitting electrode and at least one receiving electrode that are capacitively coupled by a conductive body. The receiving electrodes are connected to an analysis circuit that determines the capacitive coupling of the transmitting antenna with the conductive body by comparing the measured signal with a reference signal.
Various other systems have been disclosed with electrodes arranged at different locations in the passenger compartment in order to detect the presence and/or the nature of seat occupancy and to classify the occupancy status in one of several classes.
European patent application EP 1 457 391 A1 discloses a system for the detection of seat occupancy in a vehicle, which comprises a capacitive seat electrode and a capacitive foot-area electrode arranged in a compartment of the vehicle. During operation, capacitive coupling between the seat electrode and an object placed on the seat is determined, as well as capacitive coupling between the foot-area electrode and the seat electrode. The seat electrode comprises a shielding electrode (guard electrode), directed towards the seat frame, and a sensing electrode, directed towards the occupant of the seat. An insulating layer is arranged between the sensing electrode and the shielding electrode. In operation, the shielding electrode and the sensing electrode are driven by the same signal, so that the guard electrode prevents the electric field from the sensing electrode to couple with the seat frame. Thus the sensing electrode is active only in the direction of a seat occupant and not towards an object placed below the seat.
Integration of electrodes into the compartment of a vehicle implies certain requirements to the configuration of the electrodes in terms of mechanical robustness, protection against humidity or water, and comfort.